JEE Advance - Physics (2008 - Paper 2 Offline - No. 11)
STATEMENT 1 : It is easier to pull a heavy object than to push it on a level ground.
and
STATEMENT 2 : The magnitude of frictional force depends on the nature of the two surfaces in contact.
Vysvětlení
Case I : Free body diagram.
Frictional force ($$fr$$) = $$\mu$$N
Normal force (N) = N$$_1$$ + $$mg$$
Frictional force ($$fr$$)$$_\mathrm{I}$$ = $$\mu$$(N$$_1$$ + $$mg$$) ..... (i)
Where N = Normal force
$$\mu$$ = Coefficient of friction
$$m$$ = mass of object
$$g$$ = acc. due to gravity
N$$_1$$ = Normal force i.e., vertical component of force due to push.
N$$_2$$ = Parallel component of force due to push.
Case II : Free body diagram (Pull)
For balancing the force,
N = N$$_1$$ $$-$$ $$mg$$
Frictional force $$(fr)$$$$_\mathrm{II}$$ = $$\mu$$N ..... (ii)
From eq. (i) and (ii)
$$(fr)$$$$_\mathrm{I}$$ > $$(fr)$$$$_\mathrm{II}$$
In case of pull, it is easy to move object.
Statement 1 is true. Statement 2 is true.
Force of friction depends on roughness of two surfaces in contact. If roughness will increase, the force of friction will also increase.